Management of Sweetpotato Whitefly Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) on tomato using biorational pesticides (Neem, Abamectin and Spinosad) and UV-absorbing nets and films as greenhouse cover in the humid tropics
Von der Naturwissenschaftlichen Fakultt der Universitt Hannover zur Erlangung des Grades
Doktor der Gartenbauwissenschaften - Dr. rer. hort. -
genehmigte Dissertation von
M.Sc. Prabhat Kumar geboren am 27. December 1970 in Muzaffarpur, India
Prof. Dr. Hans-Michael Poehling
Prof. Dr. Hans-Juergen Tantau
Tag der Promotion:
---- Dedicated to my late grandparents ----
Sri. Ramnandan Mishra ji & Smt. Ahilaya Devi
The sweetpotato (Whitefly, WF) Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) originates from tropical and subtropical regions, now having a worldwide distribution as a serious pest of open field vegetable production (Tropics, Sub-tropics and Mediterranean regions) and crops grown under protected cultivation. The short and multiple life cycles with high reproduction rates under tropical conditions, fast selection of resistant biotypes to different classes of insecticides including organophosphates, pyrethroids, cyclodiens and even first, second generation neurotoxin nicotinoids, and even growth regulators are major control constraints. In addition, the waxy shelters protecting the immobile larval and pupal WF stages, high immigration and generation time, wide range of hosts (over 600 plant species) are characteristics that make its control extremely difficult. Subject of the present studies were exploring the potential of the botanical pesticides, neem using its various application methods and concentrations to control WF and evaluating its persistency compared to so-called bio-rational natural pesticides like spinosad and abamectin. In addition, physical control strategy by using a combination of UV-blocking nets and plastics were explored to learn their potential to manipulate the immigration behavior (entry) of WF and other small sucking insect-pest of tomatoes like thrips and aphids taking into consideration also the thrips related spread of a tospovirus. In first series of experiments, neem was tested using three different treatment methods (seed, soil and foliar) and two different commercial neem products (NeemAzal T/S 1% Azadirachtin and NeemAzal U 17% Azadirachtin) against WF on tomato plants. Studies were conducted in cages in air conditioned cultivation rooms. All three methods of neem treatments resulted in reduced colonization and oviposition by WF. Overall oviposition intensity was significantly reduced by the treatment of tomato seeds (261 eggs in control compared to 147 eggs at a dose-rate of 3.0g/l of NeemAzal U) but an even higher reduction was achieved through soil drenching (345 egg in control compared to 90 eggs at 3.0g/l of NeemAzal U) and foliar spraying (286 eggs in control compared to 53 eggs at 10 ml/l of NeemAzal) TS. In contrast, in soil and foliar treatment fecundity per female increased at highest tested concentrations (from 19 eggs/female in blank treatments to 28 eggs per female
at 3.0 g/l NeemAzal U and from 15 eggs/female to 22 at NeemAzal TS at 10 ml/l in foliar treatment). Reduced egg hatch could be observed only at high neem concentrations; 62 and 51% of deposited eggs hatched at highest doserates of NeemAzalU at 3.0 g/l in case of seed and soil drenching treatments respectively; whereas only 43% of deposited eggs hatched in case of foliar treatments at highest dose-rates of 10 ml/l using NeemAzal T/S. Seed (35%), foliar (93%) and soil treatments (91%) caused a significantly higher mortality of immatures and reduced number of hatching adults compared to control plants treated with a blank formulation or water. The mortality amongst immatures increased in relation to azadirachtin concentrations. Concerning susceptibility of different developmental stages, young larvae showed the most sensitive reaction. The most efficient treatment was foliar treatment, which achieved 100 % mortality for all three larval stages at high concentrations (10.0 ml/l of NeemAzal T/S) compared to 78-87% mortality with soil treatment (at 3.0g/l of NeemAzal U). To further explore the possibilities of developing synergy with locally available parasitoids of WF, persistence of foliar and systemic application of azadirachtin was tested for 7 days (1,3,5 and 7) in air conditioned rearing rooms and tropical netted greenhouses using the same two products described for the first experiments. Foliar application induced under closed room conditions at doserates of 7 and 10 ml NeemAzalTS/l immature mortality of 32 and 44 % respectively 7-days post application, where as under greenhouse conditions these rates declined to 5 and 7 % during the same period indicating rapid dissipation of active ingredient. However, systemic application resulted in more stable effects under both laboratory and greenhouse conditions. After soil drenching with solutions of 3.0 g NeemAzalU/l until 7-d, immature mortality declined from 88% for the first day to almost half (45%) on 7-d. However in case of laboratory, it was 90% on first day and declined to 64% on 7-d post application. Similar trends of responses of the B. tabaci were obtained for other parameters like adult colonization, egg deposition and egg hatch. The loss of efficiency of the neem products was clearly related to the dose-rate, methods of application and environment (temperature and UV). Soil application is therefore a convenient approach to achieve high efficiency and persistence with neem products under the critical conditions in tropical greenhouse environments.
In third experiments, direct and residual toxicity of NeemAzal TS (azadirachtin), spinosad (Spinosyne) and abamectin (Avamectin) were tested against different life stages of WF under laboratory conditions and in a tropical net greenhouse. NeemAzal TS and abamectin deterred the settling of adults on the plant and consequently reduced egg deposition. No such effect was detected for spinosad. All three pesticides influenced egg hatch. Effects of NeemAzal TS were significantly altered if applied to different aged eggs (1, 3, and 5-d old). In contrast, abamectin treated eggs failed to hatch at any given age-class. Moreover, spinosad and NeemAzal TS influenced egg hatch in a concentration dependent manner. All three products caused heavy mortality of all three larval stages of B. tabaci, where the first instar larvae was found to be most susceptible compared to other two larval stages. Larval mortalities of 100% were achieved with NeemAzal TS at twice the recommend dose-rate (10ml/l) and at all tested concentrations of abamectin and spinosad. The daily mortality rates were highest for abamectin, all treated larvae at every larval stage died within 24 h post application. In contrast, 100% larval mortality in case of NeemAzalTS and spinosad was reached 6-9 days post application. The daily mortality rates were clearly concentration dependent. Abamectin caused 100% immature mortality at all residue ages (1, 5, 10 and 15-d) in the laboratory and greenhouse as well. Persistence of spinosad was comparable high in the laboratory but in the greenhouse a faster decline of activity was evident by increased egg deposition, egg hatch and reduced rates of immature mortality. Toxicity of NeemAzalTS however strongly declined under greenhouse conditions with time (5-d) post application. The last series of experiments explored the possibility of integrating UVblocking nets and plastics to develop appropriate physical control strategies for WF. The studies were conducted to investigate the effect of ultraviolet blocked greenhouses made from combination of net and plastics on the immigration of three important pest of tomatoes; WF (Bemisia tabaci), thrips (Ceratothripoides claratris), and aphid (Aphis gossypii) and occurrences of viruses e.g. tospovirus. Fewer WF, aphids and thrips immigrated and consequently were trapped either, when gates kept open whole day (complete ventilation) or partially open from 6.00 10.00 (partial ventilation) in greenhouses made from the combination of UV-blocking nets and plastics compared to non UV-blocking nets and plastic
greenhouse. Similarly, significantly less number of alate aphids and adult B. tabaci/leaf were counted within greenhouses with low intensity of the UV over those with more UV light intensity. Thrips were the most occurring pests, that too were recorded significantly less under GH with lower UV-intensity and consequently significantly lower levels of leaf damage were recorded under these greenhouses. During, open gates experiments (complete ventilation), a 96-100% virus infestation was recorded under non UV-blocking greenhouses compared to 6-10% under UV-blocking greenhouses, having majority of the plants tested positive for the tospovirus, CaCV (isolate AIT). The virus spreads were remarkably delayed for several days under greenhouses with lower UV light. These results suggests that greenhouses made from the combination of the UV-blocking nets and plastics have a significant influence on the both the immigration and virus spread vectored by some of these insects. The results are discussed in context of improved management of sucking insect-pests of tomatoes in the humid tropics under protected cultivation.
Keywords: Bemisia tabaci, Biorationals, UV-blocked greenhosues
Die Weie Fliege (WF) Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) ursprnglich aus den Tropen und Subtropen stammend ist heute weltweit verbreitet und ein bedeutender Schdling im Feldgemsebau wrmerer Klimaregionen aber auch v